PURPOSE: We examined if the parameter of fetal bladder sagittal length (FBSL) could serve as a monitor of normative and enlarged fetal bladder size. MATERIALS AND METHODS: There were 76 consecutive cases examined between 1984 and 2000 that included measurement of fetal bladder size as FBSL and postnatal urological followup. Fetal images used to assess normal bladder size were derived from cases in which the bladder was normal on prenatal imaging and postnatal testing. An enlarged bladder was categorized as being greater than the 95% CI for a given gestational age (GA). The presence and extent of renal pelvic dilatation were also noted and correlated with FBSL. RESULTS: Measuring normal FBSL in 39 fetuses showed an exponential growth pattern (r = 0.76), which could be represented by the approximate linear formula FBSL = GA in weeks -5 (+/- 95% upper/lower CI = 7). An enlarged bladder was diagnosed in 37 fetuses. A dilated bladder in 9 fetuses, defined as FBSL greater than the 95% upper CI of normal (ie between GA + 2 and GA + 12), showed outcomes of posterior urethral valves, vesicoureteral reflux or a normal outcome. Megacystis in 28 fetuses, defined as FBSL greater than 10 mm larger than that of a dilated bladder (ie greater than GA + 12), showed additional outcomes of megacystis megaureter/vesicoureteral reflux or prune-belly syndrome. A normal outcome was significantly more likely in fetuses with a dilated bladder than in those with megacystis (p < or = 0.05). The incidence of azotemia in those with a dilated bladder or megacystis and pyelectasis was significantly lower than that in those with megacystis with hydronephrosis (p < or = 0.03). CONCLUSIONS: Postnatal diagnosis of fetuses that show an enlarged bladder is predicted based on whether the bladder is enlarged as a dilated bladder or megacystis and if the renal pelvis is enlarged as pyelectasis or hydronephrosis.